This project is a systematic study of 2 radiobiological phenomena of particular importance in tumors, namely, reoxygenation and reassortment. The EMT6 tumor system in BALB/c mice will be used most extensively, and data obtained with this system will hopefully be supplemented with other tumor systems. Having established the kinetics of reoxygenation after single conditioning doses of 1,000 and 2,000 rads, using the paired survival curve method, further experiments will explore the kinetics of reoxygenation after different schedules of fractionated conditioning doses. The reassortment experiments will be designed to investigate the possible changes in the ratio of cycling (P) to non-cycling (Q) cells in this EMT6 tumor system. Primary emphasis will be given to experiments utilizing autoradiography after 3HTdR infusion, to disclose alterations in the percent of cells in cycle. Such alterations will be sought after different single doses (and later, fractionated courses) and at different times following irradiation. Two other methods based upon the incorporation of the TdR analog, 5-bromodeoxyuridine, into the DNA of cycling cells will be tested: (1) The number and proportion of non-cycling cells will be determined after exposure of cell suspensions in vitro to visible light, which kills BrU labeled cells. (2) The feasibility of discriminating BrU-labeled (P) from unlabeled (Q) cells by the reduced fluorescence observed in labeled cells, which are first stained with the dye Hoechst 33258. Additional experiments will be done to determine the optimum timing of therapeutic regimens which combine radiation with one or more chemotherapeutic agents. Initially, these experiments will be done with adriamycin and 250 kV X-rays and will be focused upon the response of the KHT sarcoma. New projects to be undertaken under this grant include studies on the radiobiological effect of negative pi mesons, using the recently fabricated Stanford Medical Pion Generator. BIBLIOGRAPHIC REFERENCES: Kallman, R.F. The use of animal experiments in radiotherapy: Small animals. J. Canadian Assoc. of Radiologists, 26:15-26 (1975).